The present invention relates generally to an optical positioning apparatus, and more particularly to such an apparatus for accurately controlling the relation in position between a photomask of an exposure device and a wafer which is used to integrally form on its surface a microscopic circuit such as large scale integration (LSI).
For formation of a microscopic circuit on a semiconductor base, i.e., wafer, a selected pattern is transferred from a photomask to a surface of a wafer by means of the exposure technique. An important problem in the microscopic pattern transferring art relates to a highly accurate positioning of the photomask with respect to the wafer. For example, the positioning accuracy therebetween on manufacturing of an LSI may be required to be below 0.03 micrometers. One known positioning technique is made such that an alignment mark provided on a wafer is coincident with a reference mark of a photomask by moving a wafer-supporting stage in directions normal to the exposure optical axis with respect to the photomask. However, this conventional positioning techinique is insufficient for the formation of a microscopic circuit such as LSI because the positioning accuracy is about 0.3 micrometers. One possible solution for the formation of submicroscopic circuits may be to use the double diffraction positioning method and optical heterodyne interference positioning method such as are disclosed in "Applied Physics Letters", Vol. 31, No. 7, 1977 and Japanese Patent Provisional Publication No. 62-58628, the brief descriptions thereof will be made hereinafter. However, these techiniques have a tendency to result in deterioration of the positioning accuracy and therefore a further improvement would be required from the viewpoint of prevention of the deterioration of the positioning accuracy.